The present application claims priority pursuant to 35 U.S.C. xc2xa7119 from Finnish Patent Application No. 20001733, the entire contents of which is incorporated by reference herein.
1. Field of the Invention
The invention relates to a refractometer and, more particularly, to a refractometer comprising an optical module arranged floatingly inside a housing structure for measuring the index of refraction of a process fluid.
2. Background Information
The operational principle of a refractometer has been known for over a hundred years. Today, refractometers are rather widely used on several different fields. The range of use of refractometers include food processing industry, wood processing industry, chemical industry and different researches in general.
The operational principle of a refractometer can be described by way of principle in the following manner. A refractometer measures the refractive index of the process fluid by means of total reflection generated at the interface between the optical window and the fluid. The illuminating beam from the source of light is directed at the interface between the optical window and the process fluid. Part of the illuminating beam is totally reflected from the fluid, whereas part of it is partly absorbed into the fluid. This results in an image in which the location of the light and dark areas depends totally on the critical angle of the total reflection, and thus on the refractive index of the process fluid.
The essential aspect of refractometer measuring is the analysis of the image generated by the reflection of light. The objective of said image analysis is to find the critical angle of the total reflection, in other words the interface at which the light area of the image formed in the above-described manner changes over to a dark area.
As becomes obvious from the above-described aspects, the operation of a refractometer is based on very accurate angle measurement, because the critical angle of the total reflection is determined according to the refractive index of two materials. The problem with older refractometers has been angle variations of the optical window relative to the housing structure of the device. The angle variations are frequently due to the fact that the optical window in these devices is attached by means of a flexible sealing material. If the optical window is rigidly attached to the housing structure, the sealing material has to be very elastic, and thus certain materials with weak elasticity cannot be used. In several known refractometers, the optics and the light detector are rigidly attached to the housing, so that another problem is caused by an error in the angle measurement caused by the distortion of the housing structure.
To eliminate said drawbacks, a refractometer of a novel type has been provided, being disclosed in U.S. Pat. No. 6,067,151. The advantage of this solution is that the optical window can be attached also by using a weakly elastic seal, such as Teflon, without the accuracy of the angle measurement suffering from this at all.
The refractometer described in U.S. Pat. No. 6,067,151 functions extremely well in certain environments, but a problem is caused by measurement of aggressive fluids, for instance. Aggressive fluids include strong acids and basses, such as hydrochloric acid (HCl), hydrofluoric acid (HF), nitric acid (HNO3) and sulfuric acid (H2SO4), as well as sodium (NaOH) and potassium (KOH) hydroxides and ammonia (NH4OH). A plurality of acids and bases strongly corrode most of the structural metals, and alternative metals are expensive and difficult to be machined (such as tantalum and zirconium). Further, problems are caused in measurement of less aggressive fluids in cases where impurities and metal ions are not desirable in the process fluid. In such cases, the process surfaces of tube systems and instruments must not contain any metal part.
An object of the invention is to provide a refractometer by means of which the drawbacks of the prior art can be eliminated. This has been achieved by means of a refractometer according to the invention. The refractometer according to the invention comprises an optical module arranged floatingly inside a housing structure, which module comprises an optical window to be positioned in a process fluid, and means for forming an illuminating beam and for directing it into the process fluid through the optical window and for directing back the part of the illuminating beam that is reflected from the process fluid, and further, means for watching the image formed in said manner, whereby the optical module is arranged to be supported against the housing structure by means of sealing arranged between the optical window and the housing structure. The housing structure part in contact with the process fluid against which the optical window is arranged to be supported via sealing is formed of a material that is chemically durable, mechanically rigid and durable and has good thermal conductivity.
An advantage of the invention is, above all, the applicability of the structure to the measurement of very different fluids. The solution is also simple, which enables the use of plastic materials, for example, in such parts that are not in contact with the process fluid or are not mechanically subjected to great stress, so that the manufacturing costs remain low.